1. Field of the Invention
The present invention generally relates to a piezoelectric drive and, more particularly, toward a piezoelectric drive for producing a relative movement in a movement plane between a first body and a second body.
2. Description of Related Art
Piezoelectric drives, that is to say drives by way of piezoceramic materials that may be mechanically changed by an electrical voltage field, are suitable in particular for miniaturized applications, for example for motors with a motor volume of a magnitude of a few cubic centimeters or even with a volume that is smaller than one cubic centimeter. Further advantages of these drives are high torque at low speeds, simple controllability, smooth running, relatively simple construction, and their insensitivity to external magnetic fields as well as the fact that they themselves produce no magnetic fields.
Piezoelectric drives of the so-called standing-wave type comprise, as a driving element, at least one resonator, which usually consists of a piezoelement and a resonance body (horn) mechanically coupled to the piezoelement, wherein the piezoelement and the resonator body are matched to one another and the piezoelement is driven such that the resonator oscillates in a standing wave. The horn comprises a tapering free end that points away from the piezoelement and that advantageously lies on a point of the greatest oscillation amplitude. It appears that such horn tips, if they are pressed in a suitable manner against a movable body, may drive the movable body in a directed manner, wherein the force transmission is essentially based on a friction fit.
Such a drive is described in “Piezoelectric Actuators and Ultrasonic motors” of Kenji Uchino (Kluwer Academic publishers, Boston, Dordrecht, London 1997). This drive comprises a pair of disk-like piezoelements arranged coaxially over one another that are operated polarized in the opposite direction in a 3,3 mode. A horn connects essentially coaxially to the piezoelements. The horn tip is pressed against the surface of a body that is movably arranged parallel to this surface. It has been shown that the body may be driven in a directed manner with the help of the horn if the axis of the horn is not directed exactly perpendicularly towards the surface but forms a small, acute angle with the perpendiculars to the surface. If the resonator arranged in such a manner is operated at an eigenfrequency, it drives the body in that direction in which the slightly oblique horn tip points. The induced, directed movement of the driven body is explained by elliptical oscillations of the horn tip in a plane perpendicular to the surface of the body. A reversal of the movement direction is achieved by a reorientation of the resonator axis.
A similar piezomotor is described in the publication DE-3920726 (Olympus Optical). In place of the resonator of the motor described briefly above, which is symmetrical to its axis, the motor according to DE-3920726 comprises an asymmetrical horn whose tapering end does not lie on the resonator axis. The resonator is arranged with the axis directed perpendicularly to the surface of the body to be driven and with a suitable shaping of the horn tapering towards the horn tip, likewise results in a directed movement of the body created by an elliptical movement of this horn tip. At the same time there are oscillation conditions at frequencies different from one another that produce movements in opposite directions. The movement direction may thus be set via the frequency driving the piezoelements. The drive is suggested for application as a linear drive or as a rotational drive, wherein the resonator axis is aligned perpendicular to an end-face of the rotor (axially) or perpendicular to the outer surface of the rotor (radially).
The piezoelectric drive according to DE-3920726 may be realised with relatively simple means as a rotation motor with an end-face drive. In the embodiment with the end-face drive it is also possible with simple means to mutually pretension the rotor and resonator. It is indeed particularly these embodiment forms with an axially directed resonator axis that have their limits with regard to miniaturization, and one would like to go beyond these limits.
For motors that are to be very flat, in particular in the axial direction, it is therefore suggested (e.g. in EP-0505848, ETA SA) to use a centrally arranged, circular-disk-shaped piezoelement that may be driven in a planar mode. This piezoelement is coupled to a flat resonance body, which is arranged coaxially to the piezoelement and which comprises a plurality of asymmetrical horn tips extending radially towards an outer ring. Driven by the piezoelements, the horn tips again oscillate in elliptical movements by way of which the outer ring is rotatingly moved about the resonator in a directed manner. The described drive, although being able to be designed very thin in the axial direction, however has always an outer rotor. A pretension between the resonance body and the outer ring is not possible so that the drive reacts very sensitively to wear on the horn tips and the force able to be transmitted by friction remains limited.